The transmission of light through an optical fiber is highly dependent on the properties of the coatings applied to the fiber. The coatings typically include a primary coating and a secondary coating, where the secondary coating surrounds the primary coating and the primary coating contacts the glass waveguide (core+cladding) portion of the fiber. The secondary coating is a harder material (higher Young's modulus) than the primary coating and is designed to protect the glass waveguide from damage caused by abrasion or external forces that arise during processing and handling of the fiber. The primary coating is a softer material (lower Young's modulus) and is designed to buffer or dissipates stresses that result from forces applied to the outer surface of the secondary coating. Dissipation of stresses within the primary layer attenuates the stress and minimizes the stress that reaches the glass waveguide. The primary coating is especially important in dissipating stresses that arise when the fiber is bent. The bending stresses transmitted to the glass waveguide on the fiber needs to be minimized because bending stresses create local perturbations in the refractive index profile of the glass waveguide. The local refractive index perturbations lead to intensity losses for the light transmitted through the waveguide. By dissipating stresses, the primary coating minimizes bend-induced intensity losses.
In the manufacture of optical fibers, a fiber is drawn from a glass preform, thinned to an appropriate diameter (e.g. 125 μm), coated with primary and secondary coatings, and wound on a spool for storage. There is currently a desire to increase draw speed to improve the efficiency of fiber manufacturing. One factor that limits draw speed is the speed of forming the coatings on the fiber. The primary and secondary coatings are typically formed by first applying primary and secondary coating compositions to the glass fiber and then reacting the coating compositions to form primary and secondary coatings. Typically, the primary and secondary coating compositions include photoinitiators and radiation-curable monomers and/or oligomers, and the reaction to form primary and secondary coatings is induced by curing with radiation. As draw speed increases, the need for coating compositions having faster cure speeds becomes more pressing.